This invention relates to the recovery of molybdenum and rhenium values from molybdenite ores, and more particularly relates to a process for the production of alkali molybdate and perrhenate-containing solutions having low levels of silica.
Alkali metal carbonate fusion of molybdenite (MoS.sub.2) ores or concentrates such as is described in U.S. Pat. Nos. 3,725,524 and 3,770,869, both assigned to the same assignee as the present application, results in conversion of molybdenum, rhenium and sulfur values to water soluble alkali molybdates, perrhenates and sulfates. In addition, any silica present as an impurity in the ore is converted by excess carbonate to soluble alkali silicate, for example sodium silicate. These soluble products are then removed from the solidified fusion mass by water leaching. The resulting solution is then conventionally adjusted to a pH of about 8.5 to 9 with sulfuric acid to precipitate insoluble silica and is then filtered to remove the precipitate. Typically, 150 mg/l of silica remains in solution after filtration, which is sufficient to cause physical handling problems in subsequent solvent extraction processes to recover molybdenum from the alkali molybdate containing leach solution. In addition, this amount of silica will remain with the molybdenum throughout the solvent extraction process and result in molybdenum products which are unsuitable for certain chemical and metallurgical applications.